This invention relates to a measuring apparatus for use with an electric motor, and to a drive apparatus for an electric motor which uses a measuring apparatus in determining the angular position of the electric motor. It has particular application in an electric power steering system.
Electric motors have found application in a wide range of situations. One particular area in which they have found use in the last two decades is electric power steering systems. In such a system, an electric motor is used to provide an assistance torque for the driver. The motor is connected to the steering system and applies a torque that is dependent on the torque applied to the steering system by the driver. This is typically measured using a torque sensor connected to the steering column.
In order to control the motor, it is often required to know the position of the motor. This measurement can also be used in a steering system to determine the position of the steering column and hence the angle of the roadwheels and steering wheel. Provided that there is a fixed relationship between the motor angle and the steering wheel then knowledge of the position of one allows the position of the other to be calculated. This measurement is usually performed using an angular position sensor.
It has been appreciated that it would be advantageous to be able to measure motor position without providing a separate position sensor. This would allow sensorless control and potentially reduce both costs and complexity of the system.
WO/2004/023639 discloses a drive system for a three phase brushless motor that uses an algorithm for position sensing which measures the inductance in the motor phases and determines the position from the measured inductances. In order to reduce acoustic noise, the inductances are measured during test periods which are conducting states added to the PWM pattern in each PWM period. The test periods have net voltage of zero and therefore do not affect the output of the motor, but are sufficiently long to allow inductance measuring as well as the use of a single current sensor for current sensing.
A simple position measuring circuit for an electro-hydraulic power steering system is suggested in WO2004/023639 involving a differential amplifier 610 connected directly across a shunt resistor 620 in a ground line to measure the rate of change of current di/dt in a phase and from this determine the phase inductance and then position. The amplifier will amplify the voltage dropped across the resistor and must have a high dynamic range to accommodate the large change in voltage at the start of a test pulse as well as the small changes in voltage within a test pulse. The need for the high precision amplifier high dynamic range amplifier increases costs which the applicant now realises is undesirable. This can be seen in FIG. 1 of the accompanying drawings. In applicants later application WO2008/139718, and corresponding U.S. Pat. No. 8,240,281 B2, both of which are incorporated by reference herein in entirety, a sample and hold circuit is used to remove the DC offset, but such a solution is also necessarily complex.